The present disclosure relates in general to semiconductor manufacturing technology, and more particularly, to a method and system for improving critical dimension uniformity in a semiconductor wafer.
As the critical dimensions of semiconductor devices become smaller and smaller, the resolution of the exposure tool used to create the device becomes a more important consideration for manufacture. One way to increase the resolution is to reduce the exposure wavelength. However, reducing the exposure wavelength results in a relatively high manufacturing cost due to changes needed for masks, lenses, and resists. Another way to increase the resolution is to increase the numerical aperture of the projection lens used in the exposure tool. However, one side effect of increasing the numerical aperture of the projection lens is that the depth of focus becomes smaller as the numerical aperture becomes higher. Thus, the focus of the exposure tool becomes an important issue to control.
In order to control the focus, the focus must first be correctly determined. One method for determining focus is the introduction of phase-shift mask monitors, which includes adding a phase shifter adjacent to overlaying marks and measuring a defocus-induced image displacement on an overlay metrology tool. In addition to phase-shifters, phase grating may be used to further enhance the focus sensitivity. While these methods provide focus monitoring, special mask types are required to achieve a desired specific imaging property. Thus, it is costly to use these techniques in a production environment.
A need exists for a method and system that provides a model-based focus monitoring technique from within the process tool which reflects true production process conditions regardless of mask types.